One of the major processes used in the foundry industry for making metal parts is sand casting. In sand casting, disposable foundry shapes (usually characterized as molds and cores) are made by shaping and curing a foundry mix which is a mixture of sand and an organic or inorganic binder. The binder is used to strengthen the molds and cores.
Two of the major processes used in sand casting for making molds and cores are the no-bake process and the cold-box process. In the no-bake process, a liquid curing agent is mixed with an aggregate and shaped to produce a cured mold and/or core. In the cold-box process, a gaseous curing agent is passed through a compacted shaped mix to produce a cured mold and/or core. Polyurethane-forming binders, cured with a gaseous tertiary amine catalyst, are often used in the cold-box process to hold shaped foundry aggregate together as a mold or core. See for example U.S. Pat. No. 3,409,579. The polyurethane-forming binder system usually consists of a phenolic resin component and polyisocyanate component which are mixed with sand prior to compacting and curing to form a foundry mix.
Among other things, the binder must have a low viscosity, be gel-free, remain stable under use conditions, and cure efficiently. The foundry mix made by mixing sand with the binder must have adequate benchlife or the mix will not shape and cure properly. The cores and molds made with the binders must have adequate tensile strengths under normal and humid conditions, and release effectively from the pattern. Binders which meet all of these requirements are not easy to develop.
Certain modified polyisocyanates are known in the prior art. For instance U.S. Pat. No. 4,396,738 discloses modified polyisocyanates prepared by the partial reaction of some of the isocyanate groups of the polyisocyanate with a monohydroxy alcohol having the formula ROH, where R is a hydrocarbon containing six to thirty carbon atoms. These modified polyisocyanates are combined with vinyl lattices and used in aqueous coatings and adhesives.
U.S. Pat. No. 3,645,942 discloses prepolymers prepared by reacting a polyisocyanate with castor oil such that the ratio of OH groups to NCO groups is from 67:100 to 12:100. These prepolymers are mixed with a second mixture comprising 2,4-bis [N,N-di-2-(2-hydroxypropoxy) propyl] amino-1-methyl benzene and certain accelerators. The mixture is used for preparing foundry cores.
U.S. Pat. No. 4,507,408 vaguely alludes to the use of prepolymers of multivalent alcohols with polyisocyanates for use in the preparation of foundry binders.